This invention relates to medical apparatus for electrically stimulating body functions and, more particularly, to apparatus for performing compatibility checks between a stimulating device and the patient prior to implanting the device in the patient. In a particular embodiment, a pacer analyzer is provided for determining the electrical requirements of an electrode assembly implanted in a patient's heart, for determining the output available from a pacer which is about to be implanted in the patient, and for determining a measure of safety margin between the pacer's operating characteristics and the electrode stimulating requirements.
When it is determined that a cardiac pacer will assist in maintaining the cardiac output, a variety of implantable pacers exists to provide stimulating pulses. Many of the cardiac pacers are provided with a fixed set of operating parameters; other pacers provide for a limited adjustment in a few of the parameters. It is desirable to verify that the operating characteristics of such pacers are compatible with the patient within which the pacer is to be implanted. Still other pacers are programmable, having a variety of adjustable operating parameters. For these pacers, it is desirable to adjust the operating characteristics to match the particular patient prior to implanting the pacer.
In the course of implanting a pacer, an electrode is first inserted into the heart and the stimulating electrode tip lodged adjacent heart tissue in the ventricle and/or the atrium. The exact placement of the electrode tip and its electrical contact with the heart tissue determine the stimulating requirements for a given pacer output pulse. Thus, the compatibility between the pacer which is to be implanted and the electrodes lodged in the heart can only be determined at the time of implantation. It is very desirable to be able to measure various characteristics of the implanted electrodes and various output parameters of the pacer to obtain a check on the compatibility before actually implanting the pacer in the patient.
A typical prior art device for accomplishing this compatibility check is shown in U.S. Pat. No. 4,141,367, issued Feb. 27, 1979, to Med Telectronics, Ltd. The device depicted therein provides for obtaining measurements of external pacer characteristics, such as rate, pulse width, peak voltage, and trailing edge voltage. The pacer analyzer uses a simulated R-wave having a sin.sup.2 wave shape. Heart measurements from the implanted electrode may also be obtained, such as intrinsic or evoked R-wave voltage and impedance of the implanted electrode. Further, various internal pacer characteristics may be provided for pacing the heart, such as pulse frequency, pulse voltage, and pulse width.
In operation, the U.S. Pat. No. 4,141,367 pacer analyzer provides a variety of adjustment knobs for varying the particular parameter represented by the particular knob. Thus, a physician using the analyzer must carefully select the adjustment knob prior to making a measurement. Further, various sensitivity measurements, such as response of the external pacer to simulated R-wave inputs, must be done manually. Further, although it is stated that sequential atrial-ventricle (A-V) sequential pacing can be incorporated, there is no description of apparatus having A-V sequential pacing capability or ancillary capabilities such as measuring a pacer A-V delay or varying the internal pacer A-V delay to monitor the heart response.
It is also desirable to provide automatic reversion of the internal pacer to a nominal set of pacing conditions and in the rate controllable mode whenever the operating mode of the device is changed. In this manner, the pacer analyzer will produce pacer pulses and be ready for adjustments to correct any abnormal heart behavior which may result from the change in pacer stimulation.
The prior art device provides a switch for actually connecting the pacer to the electrodes in the heart for a final compatibility check, at the discretion of the physician. It would be desirable to obtain a compatibility check without actually connecting the pacer to the patient. Such a compatibility check is extremely desirable since the frequency components of a given R-wave vary from patient to patient, and the resulting R-wave presented to the pacer sensing circuitry after conventional filtering may be greatly altered from the signal produced by a simulated R-wave. These and other problems in the prior art device are overcome by the present invention and an improved pacer analyzer for use in providing a final compatibility check between a pacer to be implanted and the patient having implanted electrodes and usable in an operating room environment is presented.